Process control systems typically include various components for controlling various process parameters. For example, a fluid process control system may include a plurality of control valves for controlling flow rate, temperature, and/or pressure of a fluid flowing through the system. The end product is dependent on the accuracy of the control of these parameters, which is, in turn, dependent on the geometry and characteristics of the control valves. Control valves are, for example, specifically designed and selected to provide for particular flow capacities and pressure changes. When these characteristics are compromised, the quality of the end product may be affected.
Designing large control valves that are used at elevated temperatures is very difficult. As is known, a control valve can be provided with one or more seals to prevent leakage. A polymer seal can, for example, be provided between the valve seat ring and the valve body. Such polymer seals can be ineffective when used at high temperatures such as temperatures in excess of 450 degrees Fahrenheit. Moreover, when components of a large control valve (e.g., a control valve having a port size or ring diameter of at least six inches) are made from different materials, the components, by virtue of being made from different materials, have different thermal expansion coefficients, thereby producing differential thermal expansion between the components. In some cases, such as when the large control valve is used at elevated temperatures (e.g., temperatures in excess of 1000 degrees Fahrenheit), this differential thermal expansion can be quite significant. In smaller control valves (e.g., control valves having a port size of less than six inches), a single gasket can be used to compensate for any differential thermal expansion between components. The single gasket typically used in smaller control valves cannot, however, withstand the differential thermal expansion often seen in large control valves. One option is to significantly limit the number of material combinations for large control valve components. Then, however, large control valves must generally be made from components of the same material. Alternatively, costly architecture features, such as seat rings welded or bolted into the valve body, must be employed.